The ultimate objective of this investigation on the HLA-B27-related, Enterobacteriaceae-induced, reactive arthritis is to develop new modalities of treatment based on pathogenesis. Using a monoclonal antibody (Ye-2) and a series of synthetic peptides, one of the epitopes of HLA-B27 has been provisionally localized in this laboratory to a cluster of basic amino acids on the surface of its alpha-1 domain. The first specific aim is to verify the importance of these basic amino acid residues. The codons encoding these residues will be altered by site- directed mutagenesis. If some of these residues are critical, L cells expressing those mutated genes will be unable to react with the Ye-2 antibody. The second aim is to test the hypothesis that this cluster of basic residues are, similar to some of the synthetic peptides already tested in this laboratory, able to complex with lipopolysaccharides. This is important because LPS complexing can potentially induce release of arthritis-causing cytokines. The ability of complexing will be tested by photoaffinity labeling. LPS will be linked with SASD-125I, incubated with HLA-B27 (+) cells, and then irradiated. If LPS can complex with the HLA- B27 antigens, the two macromolecules will become covalently bonded and detected on autoradiography. Based on the assumption that complexing with LPS is part of the pathogenesis of arthritis the third aim of this proposal is to prepare potentially therapeutic synthetic peptides. The sequences of these will be based on the nature of the residues to be identified by site-directed mutagenesis. They will be tested for their ability to block the reactivity of the HLA-B27 with the Ye-2 antibody and with LPS. If successful, they can be used for future testing in the treatment of arthritis in HLA-B27 transgenic mice and subsequently in the arthritis patients.